1. Field of the Invention
This invention relates to solid refuse disposal, and more particularly to a vertical shaft-hearth type furnace for converting pelletized refuse into useful gaseous product and inert solid residue.
2. Description of the Prior Art
Anderson U.S. Pat. No. 3,729,298 describes a solid refuse disposal process which has as products a useful fuel or synthesis gas and an inert solid residue. The Anderson process involves the introduction of refuse into the top of a vertical shaft, refractory-lined furnace with the simultaneous introduction of an oxygen-enriched gas into the base of the furnace. The refuse forms a porous packed bed within the vertical shaft which can be analyzed in terms of three functional zones: a drying zone at the top section, a pyrolysis zone in the midsection, and a combustion or melting zone (the hearth) at the base of the furnace. Apparatus useful for practicing the Anderson process is also described in U.S. Pat. Nos. 3,801,082 and 3,985,518 to Anderson.
As the refuse descends through the shaft under the influence of gravity, it is first dried by hot rising gases which are generated in the lower shaft and hearth sections of the furnace. These gases are cooled as they give up their heat to the refuse. As the at least partially dried refuse descends further into the shaft furnace, it is exposed to still higher temperatures resulting in the pyrolysis of the organic content of the refuse. During the pyrolysis phase, the combustible organic material is decomposed in the presence of a hot oxygen-deficient (reducing) atmosphere to a solid char residue and a gaseous mixture consisting predominantly of carbon monoxide, hydrogen and a variety of hydrocarbons. The gaseous mixture rises from the pyrolysis zone while the char and remaining non-volatile inorganic materials descend into the combustion and melting zone or hearth.
In the hearth, the char, which is composed primarily of fixed carbon and ash, is reacted exothermically (combusted) with an oxygen-enriched gas which is fed into the hearth through a plurality of tuyeres. As described in the afore-mentioned Anderson prior art, the tuyeres are radially oriented and are positioned in the lower half of the hearth, just above the slag pool. The heat generated by this exothermic reaction melts any inorganic materials which are present so as to form a molten slag which is continuously tapped or removed from the base of the furnace. The hot gaseous product produced by the exothermic reaction (combustion), consisting primarily of a mixture of carbon oxides, rises into the pyrolysis zone and drives the endothermic reactions occurring therein.
One problem encountered during the initial large scale practice of the Anderson process with shredded refuse was excessive compaction of the vertical refuse bed. Such compaction leads to uneven gas flow through the bed and an attendant decrease in the overall efficiency. It was subsequently discovered that by compacting the refuse into small pellets for feeding into the vertical shaft furnace, many of the problems encountered when operating with shredded refuse could be eliminated. This so-called pelletized refuse process is described and claimed in Anderson U.S. Pat. No. 4,042,345 and significantly improves the overall operation of the basic process.
Even though the pelletized refuse modification of the basic Anderson process has yielded a refuse disposal system that effectively achieves the basic goals of the original process, i.e., the continuous production of a fuel gas and a clean, inert slag residue, one problem still remains. During extended operation of the Anderson pelletized process, it has been observed that some of the char produced within the pyrolysis zone is not sufficiently combusted within the hearth and is entrained in the upwardly flowing gases. This char is accordingly carried out of the vertical shaft furnace with the product gas. While this char entrainment problem does not appear to deleteriously affect the basic function of the process, it does add considerable expense to the system by complicating the fuel gas cleaning apparatus and requiring an additional sub-system for the recycle of the char back into the shaft furnace. Therefore, it would be desirable to reduce or preferably eliminate the problem of char entrainment.
An object of the present invention is to provide a vertical shaft-hearth type furnace for pelletized refuse which reduces the overall level of char entrainment in the gaseous product.
Another object of this invention is to provide a refuse disposal furnace having an improved hearth configuration which provides more complete combustion of the char produced in the pyrolysis zone, thereby contributing to a reduction in char entrainment.
Other objects and advantages of this invention will be apparent from the ensuing disclosure and appended claims.